The present invention is directed generally to the fabrication of rotary inertial energy storage devices, and more particularly to an improved rim for use in such energy storage devices. This inventon was made in the course of, or under, a contract with the U.S. Department of Energy.
Rotary inertial energy storage devices are receiving increasing interest in applications where energy storage can be beneficially and efficiently utilized. These devices or flywheels are generally formed of a centrally disposed hub which is fixed through suitable drive means to an energy input means for rotating the flywheel to suitable energy storage speeds and to a point of use for the stored energy. Circumferentially spaced from and concentrically disposed from the hub is a rim which is the mechanism for storing the inertial energy. This rim may, in turn, be coupled to the hub by a suitable spoke arrangement such as provided by windings or belts of filamentary material. The flywheel technology has significantly advanced during recent times especially in the area of rim fabrication wherein anisotropic filamentary materials, such as fiberglass, carbon and polyaramid fibers, have been successfully utilized and which offer significant advantages since they have a substantially greater strength-to-density ratio than high grade steel utilizable in flywheel construction. Filamentary materials of the aforementioned type are normally wound about a mandrel and bonded together into a single body to define the rim. However, while the filamentary materials can sustain high stresses along their longitudinal axis, significant stresses are encountered in directions perpendicular to the longitudinal axis of the filamentary material during the rotational movement of the flywheel which may cause premature failure of the rim. The stress on a free thin hoop of the wound filament varies as the square of the distance of the filament from the center of rotation with the strain being proportional to this stress. In a thick rim the outer filaments are being held back by the inner filaments while simultaneously the inner filaments are being pulled out by the outer filaments. This produces radial tensile stresses in the rim which can not be reliably supported by most filamentary composites. For example, with a rim having an inside radius of about three-fourths of its outside radius, the outermost filaments in their "free" state would strain approximately 1.8 times as much as the filaments on the inside radius of the rim. With rims of greater radius ratios the strain ratio of the outer filaments is even greater so as to induce considerably more radial tensile stress through the rim wall. This difference in strain of filaments at these locations on a rim causes the filament wound flywheel to delaminate and, in effect, break into a plurality of concentric rings at a rotational velocity considerably less than that which the ring can reach before the filaments reach their maximum tensile stress.